NewRadX is missing a README file

[Roland Haas]

All Einstein Toolkit thorns should have a readme file that lists authors, maintainers and license. This readme file must conform the the format created by the make newthorn target to be parsable by the website.

Without it the thorn should not pass review: https://docs.einsteintoolkit.org/et-docs/How_to_Review_a_new_component#Formal_criteria

[Cheng-Hsin Cheng]

I made a pull request to add a README file to the main branch for SpacetimeX.

[Roland Haas]

Applied as git hash 0e869858 "Merge pull request #26 from chcheng3/main" of SpacetimeX

[Roland Haas]

Thank you.